The present invention concerns data transmission techniques, in particular data transmission techniques in which the transmitted data is structured in blocks known as frames comprising payload data and data referred to herein as auxiliary data enabling frame synchronization of a receiver to a transmitter.
Auxiliary data of the above kind generally includes a synchronization pattern (flag) formed of a predetermined sequence of bits usually placed at the head of the frame.
Because it is necessary to guard against the payload data imitating the synchronization pattern, auxiliary data of the above kind usually further comprises synchronization bits intended to prevent such imitation interleaved with the payload data: if the synchronization pattern is a sequence of eight bits at 0, for example, bits at 1 are inserted every seven bits.
However, inserting auxiliary data of the above kind is clearly to the detriment of the payload data, which therefore raises a problem of frame efficiency, especially if it is required to increase the transmitted payload without increasing the bit rate.
To this end, the present invention consists in method of transmitting data frames including payload data and auxiliary data enabling frame synchronization of a receiver to a transmitter, said method being essentially characterized in that:
it has two transmission modes:
a first mode used on initialization or in the event of loss of frame synchronization by a receiver referred to as a first receiver whereby a transmitter referred to as a first transmitter sends to said first receiver frames including sufficient auxiliary data to enable said first receiver to acquire said frame synchronization in a reliable fashion, said auxiliary data including a synchronization pattern,
a second mode used otherwise whereby said first transmitter sends frames including insufficient auxiliary data to enable said first receiver to acquire said frame synchronization in a reliable fashion but, by virtue of knowledge of said frame synchronization acquired previously, in said first mode, enabling loss of frame synchronization to be determined,
loss of frame synchronization by said first receiver is signalled to said first transmitter by a second transmitter associated with said first receiver in a common transmission equipment changing from said second transmission mode to said first transmission mode and detection of said change of transmission mode by a second receiver associated with said first transmitter in the same transmission equipment, and
said second transmitter transmits modified frames in said second mode including no payload data sequence imitating the synchronization pattern of the first mode, also referred to as illegal sequences, which are replaced by substitution data enabling the receiver to insert said illegal sequences into the received payload data.
Accordingly, in the event of initialization or of loss of synchronization, the first transmission mode is used to enable the receiver to acquire frame synchronization in a reliable manner and other than in either of the above two cases the second transmission mode is used so that the payload transported can be increased, the system reverting to the first mode as soon as loss of synchronization is detected in the second mode.
Furthermore, any problem due to a risk of erroneous detection of this change from the second mode to the first mode itself due to a risk of imitation of the synchronization pattern of the first mode by the payload data transmitted in the second mode is avoided by the transmission of said modified frames in the second mode.
The present invention also consists in a corresponding transmission device.
The present invention also consists in a modified frame of the above kind.
The present invention is applicable to frames like V110 frames for transporting data at bit rates that are not sub-multiples of 64 kbit/s, for example those obtained for the data transmission services offered by the GSM (Global System for Mobile Communications) network, in 64 kbit/s channels.